High voltage stabilization circuit for video display apparatus

ABSTRACT

In a television receiver, an ultor accelerating potential or high voltage for a picture tube is derived by rectifying a retrace pulse voltage developed in a high voltage winding of a flyback transformer. The retrace pulse voltage is developed by a horizontal deflection circuit output stage that is coupled to the high voltage winding via a primary winding of the flyback transformer. The horizontal deflection circuit output stage includes a horizontal deflection winding, a retrace capacitor and a trace switch. The trace switch includes a damper diode and a horizontal output transistor. An energy storage coil is coupled in parallel with a third winding of the flyback transformer during a controllable time interval beginning during the second half of horizontal trace and ending during retrace. The energy stored in the coil during the trace portion is transferred to the flyback transformer during retrace to enhance high voltage at high beam currents. The amount of energy is controlled or regulated to obtain a stabilized high voltage. The energy storage coil is parallel coupled to a retrace resonant circuit during the energy transfer interval and thus stabilizers also the horizontal retrace time which might have otherwise increased with increasing beam current.

This invention relates to a power supply for a television apparatus withhigh voltage stabilization

In television receiver or monitor circuits, the ultor acceleratingpotential or high voltage for a picture tube is, typically, derived byrectifying a retrace pulse voltage developed in a high voltage windingof a horizontal output flyback transformer. The retrace pulse voltage isdeveloped by a horizontal deflection circuit output stage that iscoupled to the high voltage winding via a primary winding of the flybacktransformer. The horizontal deflection circuit output stage comprises ahorizontal deflection winding, a retrace capacitor and a trace switch,comprising a damper diode and a horizontal output transistor.

In typical television receiver circuits, raster size is inverselyproportional to the square root of the ultor accelerating potential.Because the high voltage circuit exhibits a certain amount of sourceimpedance, increasing the load current drawn from the ultor terminalwill result in a decreased ultor accelerating potential. Ultor voltagevariations resulting from variation of beam current occur mainly due toa leakage inductance between the high voltage and the primary winding ofthe flyback transformer. Ultor voltage variations may lead to reducedperformance. The reduced performance is manifested by undesirable rastersize variations, reduced peak brightness and poor focus at high beamcurrents.

The inner and outer aquadag of the picture tube act as a capacitancethat is charged by flyback transformer current, during retrace, and isdischarged by the beam current. Increasing beam current requiresincreasing charge current, during retrace. This leads to increasedloading of the deflection retrace circuit that includes a deflectionwinding, causing an increased retrace time width and reduced retrace orflyback voltage pulse amplitude. The result is a further reduction ofperformance because of "S" shaping variation due to retrace time widthvariation as a function of the beam current.

The introduction of very large picture tubes and in particular theintroductin of the 16:9 aspect ratio picture tubes may require improvedperformance of the high voltage and deflection circuits. For example,the display of a 4:3 picture on a 16:9 picture tube will show the leftand right hand picture borders. Any display breathing or deflectiondisturbance due to retrace voltage amplitude and/or width variation maybe visible because the masking overscan is missing. It may be desirableto improve stability of high voltage and retrace width with beam currentvariation.

In accordance with an aspect of the invention, an energy storage coil iscoupled in parallel with a winding of a flyback transformer during acontrollable time interval beginning during the second half ofhorizontal trace and ending during retrace. The energy stored in thecoil during the trace portion is transferred to the flyback transformerduring retrace to enhance high voltage at high beam currents. The amountof energy is controlled or regulated to obtain a stabilized highvoltage.

In accordance with another aspect of the invention, the energy storagecoil is parallel coupled to a retrace resonant circuit during the energytransfer interval and thus stabilizes also the horizontal retrace timewhich might have otherwise increased with increasing beam current.

A regulated power supply embodying a further aspect of the invention,includes a retrace resonant circuit that includes a deflection winding.A deflection current is generated in the deflection winding, during adeflection cycle and a first pulse is generated in a first winding of aflyback transformer, during retrace. A pulse-modulator is responsive toa control signal for generating a pulse-width-modulated signal that ismodulated in accordance with the control signal. A switching arrangementgenerates a second pulse in the second winding, during retrace, that ismodulated in accordance with the modulated signal. Both the first andsecond pulses are transformer-coupled to a load circuit via thetransformer for generating one of a regulated load voltage and aregulated load current in the load circuit.

FIG. 1 illustrates a horizontal deflection circuit with ultor voltageregulation circuit with ultor voltage sensing, embodying an aspect ofthe invention;

FIGS. 2a-2e and 3a-3f illustrate waveforms useful for explaining theoperation of the circuit of FIG. 1;

FIG. 4 illustrates a second embodiment of the invention in which anultor voltage regulation circuit senses directly a transformer current;and

FIG. 5 illustrates a third embodiment of the invention that includes araster distortion correction circuit.

FIG. 1 illustrates a horizontal deflection circuit 100 and a highvoltage stabilization or regulator circuit 102, embodying an aspect ofthe invention, that generates a stabilized ultor voltage U. Thearrangement of FIG. 1 may be used in conjunction with, for example, a37" color picture tube, not shown, of the type Mitsubishi A89JKA81X. Forsimplicity, east-west raster correction, horizontal linearity correctionand component values, which are not relevant for explaining theinvention, are omitted from FIG. 1.

A switching transistor Q1 of deflection circuit 100, responsive to ahorizontal rate drive signal, generates a horizontal rate retracevoltage V1. Voltage V1 is generated in a deflection retrace or flybackresonant circuit 79. Voltage V1 is coupled via a primary winding W1 of aflyback transformer T1 to a high voltage winding W2 to form a horizontalrate retrace or flyback high voltage VW2 in each winding portion ofwinding W2. Circuit 79 includes a deflection winding LH in which adeflection current iy is generated. Rectifying diodes, DHV, coupled in adiode-split configuration to the winding portions of winding W2, producean ultor voltage U that is coupled to the anode of the picture tube, notshown.

FIGS. 2a-2e and 3a-3f illustrate waveforms useful for the explanation ofthe circuit of FIG. 1. Similar symbols and numerals in FIGS. 1, 2a-2eand 3a-3f indicate similar items or functions. The waveforms of FIGS.2a-2e are drawn in solid lines for 0.2 mA average beam current beam andin broken lines for 1.2 mA average beam current.

The waveforms on the left hand side of FIGS. 2a and 2b, referred toherein as left FIGS. 2a and 2b, respectively, are used to explain theoperation when high voltage regulator circuit 102 is disabled. In thiscase, retrace voltage V1 of FIG. 1 decreases at high beam current, asshown in broken line, and the retrace time tends to increase. Aninterval tc indicates the conduction interval of high voltage rectifyingdiodes DHV integrated in winding W2 of flyback transformer T1. Intervaltc is negligibly small at low beam currents, as shown in left FIG. 2a,but increases at high beam currents because of the leakage inductancebetween windings W1 and W2 of FIG. 1. As a result, ultor voltage Udecreases significantly from 28.5 to 25.9 KV. The high voltage level Uis equal approximately to the retrace voltage at the center of intervaltc, at left FIG. 2a. The slopes of a primary current i1 of left FIG. 2bare determined by a supply voltage B+ of FIG. 1 that is coupled towinding W1, the inductance of winding W1 and the retrace resonantfrequency of circuit 79. Voltage B+ is coupled to the undotted terminalof winding W1. Voltage B+ is supplied by a voltage regulator 66.

A negative portion of current i1 representing recovered energy flowsback to voltage regulator 66 that regulates voltage B+. Current i1increases to a peak amplitude of 1.7 A at high beam current. Theincrease appears as an added D.C. component causing the negative portionof current i1 to decrease. When the negative peak of current i1 reacheszero, no energy is recovered and a damper diode DQ1, coupled in parallelwith transistor Q1, cannot conduct. Excessive D.C. component may lead toan improper operation causing deflection distortions and a reduction ofthe high voltage.

In accordance with an inventive feature, high voltage regulator circuit102 includes an energy storage coil or inductor L1, a diode D1 and awinding W3 of transformer T1 coupled in series with a collector-emittercurrent path of a switching transistor Q2 that is controlled by acontrol circuit 103. Diode D1 is conductive during a controllableportion of trace. A snubber network that includes a capacitor C7 and aresistor R20 across coil L1 prevents excessive ringing when diode D1cuts-off during the retrace interval. The waveforms of right FIGS. 2a-2eare used for explaining the normal operation of high voltage regulatorcircuit 102. Advantageously, winding W3 of FIG. 1 is also employed in aretrace voltage power supply that includes a rectifier D2, a currentlimiting resistor R4 and a filter capacitor C2 to obtain a supplyvoltage in capacitor C2 for energizing video output amplifiers, notshown.

A high voltage representative voltage is developed at the emitter oftransistor Q3 that is coupled to a voltage divider. The voltage dividerincludes a resistor R.1 and a bleeder resistor BLEEDER. A controlcircuit 103 generates a base drive voltage V3 of transistor Q2, having awaveform of FIG. 2d. A leading edge LE of voltage V3, that causestransistor Q2 to begin conducting is phase-modulated in accordance withthe voltage across resistor R1, in an interval t1-t2 that occurs duringtrace. The voltage across resistor R1 varies when ultor voltage Uvaries. Transistor Q2 of FIG. 1 conducts at low beam current or at highultor voltage U, during an interval t2-t6 of FIG. 2d, as shown in solidline. Increasing beam current or a decrease in voltage U causes leadingedge LE of voltage V3 to advance from time t2 toward time t1, as shownin broken line that corresponds to a 1.2 mA beam current. Transistor Q2conducts during interval t1-t6 and clamps a voltage at an undottedterminal of winding W3 to ground potential. A negative trace voltage V2of FIG. 2e at a dotted terminal of winding W3 produces an up-rampingcurrent i2 of FIG. 2c that flows from ground through inductor L1, diodeD1, winding W3 and transistor Q2.

Current i2 reaches its peak amplitude at time t3, the beginning of theretrace interval. Current i2 and the impedance of coil L1 are reflectedin primary winding W1, according to the winding ratio of windings W3 andW1. Current i1 at right FIG. 2b increases at a higher rate than in leftFIG. 2b, during interval t1-t3, because the transformer-coupledinductance of coil L1 reduces the inductance of primary winding W1.Current i1 reaches a higher peak value, shown in broken lines at rightFIG. 2b, than at left FIG. 2b due to the transformer-coupled current i2of FIG. 2c. The peak amplitude of current i2 at time t3 determines thestored energy in coil L1. By transformer coupling, the stored energy isalso indicated by the difference of the peak amplitudes between currenti1 in left FIG. 2b and current i1 in right FIG. 2b.

The magnetic energy in each of transformer T1, coil L1 and deflectionwinding LH is transferred as a retrace current flow into a capacitor CRof circuit 79 of FIG. 1 during the first half of the retrace interval toproduce a retrace voltage V1 of right FIG. 2a. Current i2 flowingthrough winding W3 is a down-ramping current because of the positiveretrace voltage V2 at the dotted terminal of winding W3. Current i2reaches a zero level at time t5 of FIG. 2c. Diode D1 of FIG. 1 is thenreverse biased by retrace voltage V2 and decouples coil L1 from windingW3. Thus, coil L1 is parallel-coupled to retrace circuit 79, as long asdown-ramping current i2 flows.

A trace voltage VCS, developed across a trace capacitor CS that iscoupled in series with deflection winding LH, has a D.C. voltagecomponent that is equal to voltage B+. The D.C. voltage component ofvoltage VCS is substantially unaffected by variation of the conductioninterval of transistor Q2 of circuit 102. Deflection current iy isregulated in accordance with the D.C. voltage component of voltage VCSthat is determined by voltage B+. Voltage B+ is regulated independentlyof the operation of high voltage regulator circuit 102.

In accordance with an inventive feature, the negative feedback loop thatincludes circuit 102 regulates ultor voltage U without significantlyaffecting deflection current iy. Thus, a change in the peak amplitude ofcurrent i2 caused by a change in beam current loading does not affectcurrent iy that is regulated separately by voltage B+.

In accordance with another inventive feature, the retrace frequencyincreases during current i2 conduction interval, t3-t5 of FIG. 2c, tocompensate for retrace time modulation that is caused by beam currentvariations. Therefore, voltage V1 of right FIG. 2a increases faster athigh beam current than at low beam current, as shown by the differencebetween the solid and broken lines. The additional energy transferredfrom coil L1 of FIG. 1 produces a higher peak amplitude of voltage V1 athigh beam current than at low beam current. As a result, advantageously,the decrease in high voltage U is smaller at high beam current and theretrace time remains constant.

At high beam currents, the level of the high voltage U is approximatelyequal to an average value of pulse voltage V1, as shown in the brokenline of FIG. 2a, that occurs during interval tc when rectifying diodesDHV of FIG. 1 are conductive. The average value of voltage V1 duringinterval tc is approximately equal to the peak value of voltage V1 atlow beam current. The difference between voltage U at high and low beamcurrents at right FIG. 2a is, advantageously, much smaller when comparedwith that shown in left FIG. 2a.

Circuit 103 produces pulse-width-modulated voltage V3 that controlstransistor Q2. In circuit 103, a low voltage end of bleeder resistorBLEEDER is coupled to a filter capacitor C1, and via, resistor R1, to anemitter of a transistor Q3. A biasing network that includes a resistorR7, a diode D4, a zener diode D5 and a resistor R8 provides a stablereference base voltage of transistor Q3 and an emitter voltage of atransistor Q4.

A bleeder current in resistor R1 is split into a greater portion thatflows through a series arrangement of a resistor R2, a resistor R3, aresistor R5 and a flyback transformer winding, not shown, of transformerT1 to ground. That flyback winding in the current path of resistors R2,R3 and R5 generates a 250 Vpp negative going retrace pulse at a terminal103a. For D.C. purposes, the voltage at terminal 103a represents groundpotential. The retrace pulses at terminal 103a are integrated by anintegration network that includes resistor R5 and a capacitor C3 toproduce a sawtooth voltage V4.

The other, and smaller, portion of the bleeder current flows throughtransistor Q3 and through a beam current sampling load resistor R6. Ahigh voltage representative voltage V5 developed across resistor R6 isfiltered by a capacitor C4. Whereas, voltage V4 across resistor R5 isnot modulated by high voltage variations because the emitter oftransistor Q3 is at a constant D.C. potential. Voltage V5 varies withultor voltage variations, as shown in FIG. 3b. Advantageously, thecurrent that flows through resistors R2, R3 and R5 reduces the D.C.level of voltage V5, thus enabling the usage of a higher value resistorfor resistor R6.

Voltage V4 is compared with voltage V5 by a voltage comparator U1A formodulating a trailing edge of voltage V9 during an interval t1-t2 ofFIG. 3f. Comparators U1B and U1C of FIG. 1 are driven by retrace pulsevoltage V6 to hold down output voltages V8 and V9 of comparators U1B andU1C, respectively, during retrace. A comparator U1D is driven by asawtooth voltage V7 of FIG. 3d produced by an R-C network of a resistorR10 of FIG. 1 and a capacitor C5. Output voltage V8 prevents transistorQ2 from being turned on during the first half of the trace interval.Voltages V8 and V9 are summed via resistors R16 and R15 to drive a baseof a transistor Q5. Transistor Q5 produces, at its collector, voltage V3that is also developed at the base of transistor Q2. High voltage U isadjusted by a variable resistor R3 which varies in a differential mannerthe D.C. levels of voltages V4 and V5.

Transistor Q4 provides protection by disabling circuit 103 andtransistor Q2. When the bleeder current falls below approximately 70% ofits nominal value, transistor Q3 becomes disable or nonconductive andtransistor Q4 becomes conductive. The current flowing through resistorR7, diode D4 and transistor Q4 charges capacitor C4 to a higher voltagethan voltage V4, causing the sum of voltages V8 and V9 to be positiveduring trace. As a result, transistor Q2 cannot conduct during trace andcurrent i2 is zero. Such fault condition can occur with a defective ordisconnected bleeder resistor. Advantageously, the protection operationprovides a soft start-up operation because the high voltage regulator isdisabled until high voltage U is equal to at least 70% of its nominalvalue.

FIG. 4 illustrates a high voltage regulator 102', embodying anotheraspect of the invention. Similar symbols and numerals in FIGS. 1 and 4,with the exception of the symbol (') in FIG. 4, indicate similar itemsor functions. Series coupled diode D1', inductor L1' and transistor Q2'of FIG. 4 are coupled to a winding W3' which supplies negative goingretrace pulses. Winding W3' is also used as a voltage source for a tracerectifier D2', for producing in a capacitor C2' a supply voltage of 28volts required by, for example, a vertical deflection amplifier, notshown.

The operation of a high voltage regulator circuit 102' is similar tocircuit 102 described in FIG. 1. The waveforms in FIGS. 2a-2e withrespect to the circuit of FIG. 1 are also applicable with respect to thecircuit of FIG. 4 except for the waveform of FIG. 2e which is inverted,as shown in FIG. 4. A difference between control circuit 103' of FIG. 4and circuit 103 of FIG. 1 is that control circuit 103' of FIG. 4 iscontrolled directly by a sample of the high voltage charge current inwinding W2' instead of directly by ultor voltage U, as shown on FIG. 1.The charge current through winding W2' of FIG. 4 is sampled across aresistor R21 of FIG. 4. A capacitor C8 provides filtering. The chargecurrent through resistor R21 is inverse proportional to high voltage U'.Therefore, the arrangement of FIG. 4 can operate properly in an openloop configuration. The circuit of FIG. 4 may be employed in a highvoltage circuit that does not include a bleeder resistor.

A regulation circuit 102" of FIG. 5, embodying an aspect of theinvention, provides ultor voltage regulation and operates similarly tothat described with respect to the circuit of FIG. 1. In FIG. 5, an E-Wraster distortion corrected horizontal deflection circuit 200 isincluded. Similar symbols and numerals in FIGS. 1 and 5, with theexception of the symbol (") in FIG. 5, indicate similar items orfunctions.

An E-W switching transistor Q11 of FIG. 5 is conductive and supplies acontrollable amount of energy to deflection resonant circuit 79" duringa first portion of the retrace interval for obtaining an East-Westamplitude modulated deflection current. Horizontal retrace begins whentransistor Q1" is turned off. Transistor Q11 is maintained conductivefrom a time at the beginning of the horizontal trace interval and untila controllable instant during the first portion of horizontal retrace.The retrace, first portion begins at the time transistor Q1" becomesnonconductive. The length of the first portion varies in a vertical ratemanner to provide East-West raster distortion correction. Following theretrace, first portion, transistor Q11 becomes nonconductive andisolates a flyback resonant circuit 251, that includes winding W1" and aflyback capacitance CT, from resonant circuit 79". High voltageregulation circuit 102" is also isolated from retrace circuit 79",during a second portion of the retrace interval, when diodes DHV areconductive. As a result, circuit 102" is not by-passed by circuit 79".Therefore, the efficiency of circuit 102" is, advantageously, increased.

The operation of the deflection circuit that includes circuit 79",circuit 200 and circuit 251 is described in more detail in allowed U.S.patent application Ser. No. 722,809, filed Jun. 28, 1991, entitled,RASTER DISTORTION CORRECTION CIRCUIT in the name of Haferl, that isincorporated by reference herein.

What is claimed is:
 1. A regulated power supply for a video displayapparatus, comprising:a retrace resonant circuit that includes adeflection winding; a flyback transformer; a source of an input supplyvoltage that is coupled to a first winding of said transformer; a sourceof a synchronization input signal at a frequency that is related to adeflection frequency; first switching means responsive to said inputsignal and coupled to said deflection winding and to said transformerfor generating a deflection current in said deflection winding, during adeflection cycle, and a first pulse in said first winding of saidtransformer, during retrace; a pulse-width-modulator responsive to acontrol signal for generating a pulse-width-modulated signal that ismodulated in accordance with said control signal; and second switchingmeans responsive to said pulse-width-modulated signal and coupled to asecond winding of said transformer for generating a second pulse in saidsecond winding, during retrace, that is modulated in accordance withsaid modulated signal, both said first and second pulses beingtransformer-coupled to a load circuit via said transformer forgenerating one of a regulated load voltage and a regulated load currentin said load circuit.
 2. A power supply according to claim 1 furthercomprising, an inductance coupled to said second winding to form acurrent path for producing a current in said second winding having arate of change, during trace, that is determined by a value of saidinductance and that is modulated in accordance with saidpulse-width-modulated signal.
 3. A power supply according to claim 2wherein said first switching means applies said input supply voltageacross said first winding of said transformer for transformer-couplingsaid input supply voltage to said second winding, during trace, todevelop a trace voltage in said second winding that produces in saidinductance said second winding current.
 4. A power supply according toclaim 3 wherein said second winding is energized entirely via saidtransformer.
 5. A power supply according to claim 2 wherein said secondwinding current is transformer-coupled to said load circuit, duringretrace.
 6. A power supply according to claim 1 wherein said first pulseis transformer-coupled to a high voltage winding of said transformer andwherein a rectifier is coupled to said high voltage winding forgenerating from said first pulse an ultor voltage and a beam current. 7.A power supply according to claim 6 wherein said pulse-width modulatoris responsive to one of said ultor voltage and a current that flows insaid high voltage winding for regulating said ultor voltage in afeedback manner.
 8. A power supply according to claim 1 wherein themodulation of said pulse-width-modulated signal does not affect saiddeflection current.
 9. A power supply according to claim 1 wherein saidinput supply voltage produces a current in said first winding of saidtransformer to store magnetic energy in said transformer, during trace,the stored magnetic energy replenishing energy losses in said retraceresonant circuit and in said load circuit, during retrace.
 10. A powersupply according to claim 1 wherein said pulse-width-modulated signal ismodulated, during trace.
 11. A power supply according to claim 1 furthercomprising, an inductance coupled to said second winding of saidtransformer for conducting in said inductance a ramping, second currenthaving a peak amplitude that is modulated in accordance with a beamcurrent, wherein said second switching means decouples said inductancefrom said transformer during a first portion of a retrace interval andcouples said inductance to said transformer during a second portion ofsaid retrace interval and wherein a length of each of said first andsecond portions varies in accordance with said beam current.
 12. A powersupply according to claim 11 wherein said inductance is formed in acurrent path of said second winding current and is coupled via saidtransformer to said retrace resonant circuit to vary a retrace resonancefrequency of said retrace resonant circuit in accordance with said beamcurrent in a manner to compensate for retrace time modulation that iscaused by a variation of said beam current.
 13. A power supply accordingto claim 1 wherein said second switching means comprises a two-terminalrectifier.
 14. A power supply according to claim 1 wherein saidpulse-width modulator is responsive to one of said load current andvoltage for varying a pulse-width of said modulated signal in accordancewith one of said load current and voltage.
 15. A power supply accordingto claim 1 wherein said deflection current is regulated in accordancewith said input supply voltage and is unaffected by the pulse-widthmodulation.
 16. A regulated power supply for a video display apparatus,comprising:a retrace resonant circuit that includes a deflectionwinding; a flyback transformer; a source of an input supply voltage thatis developed in a first winding of said transformer, during trace; asource of a synchronization input signal at a frequency that is relatedto a deflection frequency; first switching means responsive to saidinput signal and coupled to said deflection winding and to saidtransformer for generating a deflection current in said deflectionwinding, during a deflection cycle, and a first pulse in said firstwinding of said transformer, during retrace; an energy storageinductance coupled to a second winding of said transformer, such thatsaid input supply voltage is transformer-coupled from said first windingto said inductance, during trace, to store magnetic energy in saidinductance for generating from the stored magnetic energy in saidinductance a second pulse in said second winding, during retrace, bothsaid first and second pulses being transformer-coupled to a load circuitvia said transformer for generating one of a regulated load voltage anda regulated load current in said load circuit that is regulated inaccordance with said second pulse.